1. Field of the Invention
The present invention relates to a caliper gauge. More specifically, the present invention relates to a caliper gauge that holds an object to be measured with a constant measuring force.
2. Description of Related Art
A caliper gauge includes a main scale having a first measuring jaw, a slider having a second jaw, the slider movably provided on the main body relative to the main body, and a display unit that displays the displacement of the slider relative to the main body.
In order to measure an object, the main scale is gripped by one hand and the slider is moved along the main scale while applying thumb on the slider to bring inside or outside jaws provided on the main scale and the slider into contact with target portions of the object to be measured. The dimension and the like of the object can be measured by the value displayed on the display unit.
In such a caliper gauge, measuring force applied when the measuring jaws hold the object is sometimes of critical issue. The main scale and the object may be deformed on account of applied pressure. Especially, when the object is made of soft material such as rubber and plastic, the object is greatly deformed, which results in variation of measured value in accordance with difference in the measuring force, thereby rendering accurate measurement difficult.
Accordingly, various mechanisms have been proposed for keeping constant measuring force. For instance, a document (JP-A-6-185905) discloses a caliper gauge, in which a slider is moved along a main scale by rotating a pressing roller having a constant-pressure mechanism to avoid excessive measuring force.
In the above document, the pressing roller includes an outer roller provided on the slider, an inner roller having an outer periphery to be in contact with the main scale, and the constant-pressure mechanism that allows free rotation of the outer roller relative to the inner roller when more than a predetermined resistance force is applied on the inner roller.
When the outer roller is rotated, the rotation of the outer roller is transmitted to the inner roller through the constant-pressure mechanism. The inner roller is rotated while being in contact with the main scale, so that the slider is moved along the main scale. When the measuring jaws are in contact with the object, further movement of the slider is restricted and the outer roller is freely rotated relative to the inner roller. Excessive measuring force is thus avoided.
However, there is, for instance, the following problem in the caliper gauge of the above document.
The constant-pressure mechanism shown in the embodiment of the above document is a ratchet mechanism having a combination of a ratchet wheel and a coned disc spring. Though the ratchet mechanism keeps a constant measuring force against the rotation of the pressing roller in one direction, the measuring force cannot be kept constant against the reverse rotation. Accordingly, the above caliper gauge only keeps constant measuring force during a measurement using only one of the inner and the outer measuring jaws.